Method for detecting defects in work pieces and facility and magnetic field measuring apparatus for implementing said method

ABSTRACT

A depth and other characteristics of a ferromagnetic impurity in a workpiece of nonmagnetic material can be determined after demagnetizing of the workpiece by magnetizing the workpiece in a uniform field, preferably to saturation, and thereafter taking measurements of the field strength from the impurity at two different distances, forming a quotient or ratio of the measured values and determining the depth from a curve in which the signal ratio or quotient is plotted against the depth.

The invention relates to a process for determining ferromagneticimpurities in workpieces in nonmagnetic material, especially in wheeldisks or shafts of gas turbines for aircraft or the like. It relatesfurther to an apparatus for carrying out the process and amagnetic-field measuring device, especially as a part of this apparatus.

In machine construction practice, increasingly workpieces of componentsunder this designation of nonmagnetic materials, especially nickel-basedmaterials or titanium materials are made so that the workpieces willhave high mechanical and/or thermal loading resistances. This appliesabove all for components of aircraft gas turbines since materials usedfor them have the smallest possible specific gravity.

This applies especially to nickel-based alloys and titanium-basedalloys. In the course of metallurgical fabrication processes,iron-containing impurities enter these alloys. These impurities giverise to defects in the microstructure of the material and affect therebythe mechanism loadability of the workpiece. They are referred to asdefects and because of their iron content, as ferromagnetic defects.Depending upon the size and number of the impurities and their spatiallocations in the workpiece, the workpiece can be damaged by certainmechanical loads, leading to the destruction of the workpiece. Sinceferromagnetic impurities can always be contained in a nickel-based alloyor a titanium-based alloy, for every concrete use and loading associatedtherewith there is a certain permissible mass, i.e. size of theimpurities and their spatial positions in a workpiece which can beestablished. This maximum permissible mass of impurities is designatedas the critical mass.

Especially critical with respect to the aforementioned impurities, arecomponents or workpieces for aircraft gas turbines since, in thesecases, the durability requirements are especially high. This applieseven for the production of the respective workpieces, especially thewheel disks carrying the rotor blades or also the shafts of an aircraftturbine in which the mass of the ferromagnetic impurities in theworkpieces or a blank thereof should be detected before this workpieceis further processed and especially built into the aircraft turbine. Thelatter should take place only when the determined mass of thepossibly-present impurities is less than the above-mentioned criticalmass for such impurities.

Basically three processes for the detection of ferromagnetic impuritiesare useable, namely, X-ray tomography, ultrasonic measurements andmagnetoscope measurements.

By means of X-ray tomography, impurities can be detected based upontheir different compositions and the X-ray contrasts associatedtherewith. X-ray tomography is, however, extremely time-consuming, veryexpensive and outside routine processes in a production sequence. Themethod is in any case used for comparative measurements and forcalibrating of the processes.

An ultrasonic measurement detects impurities based upon their differentdensities. By the scattering of the ultrasound crystallites of the basicalloy, the contrast is significantly reduced. The method is thereforesuitable only for larger impurities in workpieces which are not verythick.

The magnetoscope measuring process utilizes the fact that theaforedescribed impurities are ferromagnetic. In the process, amagnetic-field measuring device with a magnetic-field sensor is usedwhich contains a magnet by means of which the impurities are magnetized.Simultaneously the measurement of the magnetic signals of the magnetizedimpurities is carried out.

The magnetoscope process has, however, in its known form, a series ofdrawbacks. Since the magnetization occurs only in the measurementregion, impurities are differently magnetized depending upon theirdepths within the workpiece. Since the magnetic induction of themagnetizing device decreases with the third power of the distance, theimpurities at the greatest distance from the magnetic-field sensor areonly extremely weakly magnetized and are thus not detected by it.Furthermore, the magnetic field as a consequence of its point-likeeffect is inhomogeneous which has, as a consequence, that the magneticsignals of the impurities differ depending upon their [the impurities]orientation with respect to the magnetizing device. Because of theunder-defined magnetizing state, conclusions cannot be made as to themass of the impurities and their depths in the workpiece.

It is the object of the invention to provide a process suitable forserial production which can enable detection of ferromagnetic impuritiesin nonmagnetic workpieces precisely, reliably and in a simple manner.

A further object is to conceive an apparatus for carrying out thisprocess and a magnetic-field measuring device associated therewith.

As far as the process goes, the objects are achieved according to theinvention in that the workpiece is subjected to a magnetic field, theimpurities at least in sections of the workpiece are uniformlymagnetized and the workpiece is thereafter fed to a magnetic-fieldmeasuring device and there the magnetic signals of the respectiveimpurity is measured.

The basic thought of the invention is that the workpiece or theimpurities, prior to the specific measurement by means of amagnetization process is brought into a magnetically-ordered and thusdefined state and only thereafter is a respective magnetic signal of animpurity detected with the aid of the magnetic-field measuring device,for example, by a suitable scanning, i.e. relative movement between themagnetic-field sensor and workpiece. Thus the requirement is satisfiedthat a particular impurity is not detected exclusively with respect toits position but also with respect to its mass in a relatively precisemanner.

It is worth carrying out the process with a through-magnetization sothat the impurities are magnetized to saturation. In most applications,however, a magnetization is sufficient which does not reach thesaturation limits for generating magnetic signals from the impurities.

In any case it is advantageous for the workpiece to be subjected to asubstantially homogeneous magnetic field. To the extent that theworkpiece is subjected at an earlier point in time to a strongermagnetic field than the magnetization field strength applied, before themagnetization [from which the signal is to be obtained], ademagnetization should be effected.

In a further embodiment of the invention it is provided that theworkpiece is scanned in a scanning plane with the magnetic measuringdevice and the position of the impurities determined with reference tothis plane in this manner. For detecting the depth of the respectiveimpurity in the workpiece, it is possible basically to scan theworkpiece in a further scanning plane, preferably lying transversely tothe first.

According to the invention however, the following features are proposedfor determining the depths of the respective impurities in theworkpiece:

(a) Detection of the signal of the impurity at a first distance betweenthe magnetic-field sensor and the surface of the workpiece;

(b) Detection of the signal from the impurity at a second distancebetween the magnetic-field sensor and the surface of the workpiece;

(c) Forming a signal ratio as a quotient of the measurement signal fromthe two detected signals from the impurity;

(d) Determining the depth based upon a curve which portrays thedependency between the signal ratio and the depth.

Thus the depth measurement is effected starting from the first scanningplane by two measuring steps at different distances from the surface ofthe workpiece to form a signal ratio and by subsequent marking off froma curve portraying the dependency between the single ratio and depth.The term “curve” is here understood in its most general form. It can bein a graphic form or in the form of a matrix or function. It will beunderstood that it is established initially by corresponding calibrationand then firmed up. It has been found that with this method in a simpleway and without large expenditure for apparatus, a relatively exactindication as to the depth arrangement of respective impurities ofconcern can be obtained.

Once the depth of the impurities is determined, mass of the impuritiescan also be established. For this purpose, the invention makes use ofthe following alternative proposals.

With one proposal, a curve is selected from a family of curves whichdisplay the relationship between mass and measurement signals at variousdepths and the mass is then established. By selection of the curve, themass can be marked off directly based upon the measurement signal.

Alternatively, the mass of the impurity can also be established bydevelopment of a family of curves which display the relationship betweenthe depth of the impurities and their measurement signals for differentmasses. Based upon the measurement signal and the determined depth, acurve can be selected which represents the respective mass of theimpurity.

The second part of the object of the invention is achieved, inaccordance with the invention with an apparatus for carrying out theaforedescribed process and which is characterized by the followingfeatures:

(a) A magnetization device for substantially uniformlythrough-magnetizing a workpiece by generating a substantially homogenousmagnetic field whether in sections or over the whole [of theworkpiece]—and

(b) A magnetic-field measuring device for detecting magnetic signals ofan impurity.

It will be self-understood that this apparatus can be comprised on theone hand from two separate units or on the other hand by a combinationdevice. Thus it is advantageous on the grounds indicated earlier toenable the magnetization device to be capable of through-magnetizing ofimpurities up to saturation. For the case in which the workpiece isalready premagnetized, it is advantageous that the apparatus alsoinclude a demagnetization unit so that the magnetization of theworkpiece depends upon the effect of the magnetizing device and on thepremagnetization.

The magnetic-field measuring unit itself, whether as part of theaforedescribed apparatus or independently thereof, is characterized bythe following features according to the invention:

(a) the magnetic-field sensor is movable in a scanning plane;

(b) the magnetic-field sensor in movable perpendicular to the scanningplane between two measurement positions;

(c) the magnetic-field measuring device has a device for calculating thesignal ratio of the measured signals at the two measuring points;

(d) in the magnetic-field measuring device a curve is stored whichrepresents the dependency between the signal ratio and the depth of theimpurity in the workpiece;

(e) the magnetic-field measuring device has a device for calculating thedepth of an impurity based upon the determined signal ratio and thecurve.

With such a magnetic-field measuring device, a particular impurity canbe relatively exactly localized and, in particular, by scanning in thescanning plane on the one hand and on the other by detection of themagnetic signals at two different distances to the scanning plane,whereby the scanning plane advantageously lies parallel to one of thesurfaces of the workpiece.

As already indicated above, the storage of the curve in themagnetic-field measuring device can be effected in various ways.Important only is that it can be incorporated in the calculating processat the end of which a signal corresponding to the depth of the impurityis obtained which can then preferably be digitally displayed.

For the additional determination of the mass of a certain impurity,according to the invention, two possible embodiments can be used. One ofthem is characterized by the following features:

(a) in the magnetic-field measuring device, a family of curves fordifferent depths is stored which relate the dependency between mass andmeasurement signal at different depths.

(b) the magnetic-field measuring device has a device for calculatingmass of the impurity from the family of curves by selection of the curvecorresponding to the determined depth and from the measurement signal.

Alternatively thereto, the following embodiment is provided:

(a) a family of curves for different mass is stored in themagnetic-field measuring device and relates the dependency between depthand measurement signals for different masses;

(b) the magnetic-field measuring device has a device for calculating themass of the impurity from the measurement signal and the determineddepth.

As the magnetic-field measuring device, basically all known devices fordetermining the magnetic fields can be considered. An especially goodresolution can be obtained with the aid of a second-order gradiometerwith a superconductive quantum interferometer (SQUID) on the basis of ahigh-temperature superconductor. Such magnetic-field measuring devicesare known per se as state of the art (Tavrin, Chang, Wolf and Braginski,A second-order SQUID-gradiometer operating at 77K, Supercond. Sci.Technol. 7 (1994), P. 265 to 268; Jenks, Sadeghi and Wikswo, SQUIDS fornon-destructive evaluation, Appl. Phys. 30 (1997) S. 293 to 323. Theycan also detect impurities at depths of 50 mm depending upon theirorientation and mass with precision and are suitable also for routineprocesses in the production of such workpieces.

The invention is described in greater detail with reference to anembodiment shown in the drawing. It shows:

FIG. 1 the side elevation of a disk-shaped workpiece with amagnetic-field sensor in a first measurement position;

FIG. 2 the illustration of FIG. 1 with a magnetic-field sensor in asecond measurement position;

FIG. 3 a graph illustrating the relationship between the signal ratio ofthe measurement signal from the two measurement positions and the depthof an impurity in the workpiece;

FIG. 4 a graph showing the relationship between a measurement signal andthe mass of an impurity in a workpiece at different depths;

FIG. 5 a graph showing the relationship between a measurement signal andthe depth of an impurity in a workpiece for different masses; and

FIG. 6 is a block diagram.

SPECIFIC DESCRIPTION

FIGS. 1 and 2 show in a simplified illustration, a disk-shapedrotationally symmetrical workpiece 1, which is rotatably journaled in aworkpiece holder not shown here in greater detail so as to be rotatableabout the axis 2. The rotation direction is indicted by the arrow A.

Above the workpiece a magnetic-field sensor 3 can be seen. It is part ofa magnetic-field measuring device not shown here in greater detail, inthe form of a second-order gradiometer with a superconductive quantuminterferometer (SQUID) on the basis of a high-temperaturesuperconductor. The magnetic-field sensor 3 is separate from or part ofthe magnetic-field measuring device and is movable in a horizontalscanning plane 4 whereby it allows scanning of the entire surface of theworkpiece 1 taking into consideration the rotational symmetry of theworkpiece 1 and its rotational mounting when it ishorizontally-displaceable in the radial direction, i.e. in the plane ofthe drawing. The apparatus elements required for this purpose have beenpartly omitted for clarity.

In the workpiece 1, overdimensionally-enlarged, there is a ferromagneticimpurity 5 located at a vertical distance z from the scanning plane 4and thus from the magnetic-field sensor 3. The latter is held directlyabove the impurity 5 at scanning, this position in the horizontal planebeing detected by the above-described scanning and, indeed beingestablished from the maximum of the magnetic signal obtained. Initiallythe workpiece 1 was, in a separate magnetizing device not here shown,subjected over its entire volume to a homogeneous magnetic field so thatthe ferromagnetic impurities in it reach a defined uniform magnetizationstate.

After positioning the workpiece 1, a magnetic-field sensor 3 in theillustrated first measurement position, a first measurement process iscommenced. Based upon the magnetic signal from the impurity 5, a firstmeasurement signal is detected. Thereafter, the magnetic-field sensor 3is moved by a magnitude h perpendicular to the surface of the workpiece1 upwardly into a second measurement position as is visible from thetwo. The magnetic-field sensor 3 now is at a distance z+h from theimpurity 5. By a new measurement process, a second measurement signal isobtained that, because of its greater distance form the impurity 5, isweaker than the first. It will be self-understood that in themeasurements applied to other impurities, the difference h between thetwo measurement positions is always identical.

For the determination of the depth d of the impurity 5, calculated formthe upper surface of the workpiece 1, initially a signal ratio is formedwith the quotient of the two measurement signals of the first and thesecond measurement positions. with the aid of the graph according toFIG. 3, the determination of the depth is thus possible, whereby thegraph gives the signal ratio along the abscissa and the depth along theordinate. The associated arrows give, as is usual in graphs, thedirections for values of increasing magnitude in the graph a curve 6,shown from which the relationship between the signal ratio and the depthof the impurity 5 in the workpiece 1 is given. It can be seen that thesignal ratio is reduced as the depth increases. Based upon the detectedsignal ratio, the curve 6 gives the depth d.

The determination of the mass of the impurity can be effected,alternatively by the graph according to FIG. 4, or the graph accordingto FIG. 5. Initially the method with the graph according to FIG. 4 isexplained.

In this graph, the abscissa indicates the measurement signal in a firstmeasurement position and the ordinate the mass since there is no directrelationship between the measurement signal and the mass—the measurementsignal being dependent not only from the mass of the impurity 5 but alsofrom its depth d in the workpiece—a family of curves is shown in thegraph and for example can be comprised of the three lines 7, 8, 9. Thelines 7, 8, 9 are determined for impurities of different mass at thesame depth. Line 7 with the steepest slope has a mean indicated by thetriangles, for example at 11. symbolizing a measured value forimpurities of a reduced depth. Correspondingly the intermediate line 8has a mean represented by the squares 12 whereby impurities of differentgasses in an intermediate plane are detected. The lower line 10represents the depth d. Here the measured values are indicated bypoints, for example, at 13. The depths of the impurities for determiningthe line 10 are here still greater than for the line 9.

With the aid of graph of FIG. 4, a mass e of the impurity 5 can bedetected by marking off the respective point along the line 10 which lieat the level of the measurement signal in the first measurementposition. As a result the value e is obtained for the mass.

In the graph of FIG. 5, the measurement signal is displayed as afunction of depth. For this purpose as well a family of curves existswhich represents the mass with increasing values in the direction of thearrow. In the graph, for example, four curves 14, 15, 16, 17 are shown.The measurement signal is logarithmically applied.

Each curve represents the relationship between measurement signal anddepth at a constant mass since the depth is known with the aid of thegraph according to FIG. 3 as much as is the measurement signal in thefirst measurement position, the respective curve 14, 15, 16, 17 can beobtained by marking off the abscissa and the ordinate.

In the present example, the curve 15 is the curve representing the masse. It will be understood that the aforedescribed type of determinationof the depth d of the impurity 5 and its mass e serves only to enableunderstanding of the invention. In the construction of themagnetic-field measuring device, the above-described relationship can bestored, for example, in a microprocessor after carrying out acalibration, whereby the calculation of the depth and the mass iseffected with the aid of a computer program on the basis of the storedrelationship. Thus the curves 6, 7, 8, 9, 14, 15, 16, 17 are stored asfunctions in the form of a matrix or in another way known in the art. Itwill be understood further that the computer program can enableinterpolation for values for which there are no lines in the family ofcurves of FIG. 4 for the determination of the depth d and no curves inthe family of curves of FIG. 5 for the mass 5.

In FIG. 6, the magnetic field sensor 3 is scannable over the workpiece 1and movable relative to a surface of the workpiece, as has beendescribed, for juxtaposition with the workpiece at a first distance fromthe surface of the workpiece to obtain a first signal measuring magneticfield strength derived from the ferromagnetic impurity after theimpurity has been magnetized in the magnetic field and at a seconddistance from the surface different from the first (compare FIGS. 1 and2) to obtain a second signal measuring magnetic field strength derivedfrom the ferromagnetic impurity. The magnetic field strength measurementare effected at 20. A ratio former 21 constitutes means for forming asignal ratio of the first and second signals as a quotient of themagnetic field strength at the first and second distances. Means 22determines from a curve in which the dependency of the signal ratio isplotted against impurity depth based upon the signal ratio, the depth ofthe impurity in the workpiece (see FIG. 3).

A storage 23 for the family of curves of different depths is connectedto the depths determination means 22 which also constitutes the meansfor calculating the mass of the impurity from the family of curves byselection of the curve corresponding to the determined depth and basedupon the measurement.

The storage means 23 can store a family of curves of different masses ofimpurity to give the dependency between depth and measurement signal fordifferent masses.

What is claimed is:
 1. A method of determining a depth of aferromagnetic impurity in a workpiece of a nonmagnetic material,comprising the steps of: (a) magnetizing said ferromagnetic impurityuniformly by subjecting said workpiece at least in sections to amagnetic field; (b) thereafter juxtaposing said workpiece and a magneticfield measuring sensor at a first distance from a surface of saidworkpiece to obtain a first signal measuring magnetic field strengthderiving from said ferromagnetic impurity; (c) then juxtaposing saidworkpiece and said magnetic field measuring sensor at a second distancefrom said surface of said workpiece different from said first distanceto obtain a second signal measuring magnetic field strength derivingfrom said ferromagnetic impurity; (d) forming a signal ratio of saidfirst and second signals as a quotient of the magnetic field strengthsat said first and second distances; and (e) based upon said signalratio, determining from a curve in which the dependency of the signalratio is plotted against an impurity depth, a depth of the impurity insaid workpiece from which the magnetic field strengths were measured. 2.The method defined in claim 1 wherein said impurity is magnetized instep (a) to saturation.
 3. The method defined in claim 2, furthercomprising the step of demagnetizing said workpiece prior tomagnetization of said ferromagnetic impurity in step (a).
 4. The methoddefined in claim 3 wherein said magnetic field is a substantiallyhomogeneous magnetic field.
 5. The method defined in claim 3, furthercomprising the step of relatively moving said workpiece and said sensorto scan said sensor over said workpiece.
 6. The method defined in claim3, further comprising determining a mass of said impurity by selecting acurve for determining depth from a family of curves which display arelationship between mass and measurement signal for different depthsand selecting the mass based upon the measurement signal and theselected curve.
 7. The method defined in claim 3, further comprising thestep of determining the mass of the impurity by selecting a particularcurve from a family of curves giving a relationship between depth andmeasurement signal at different masses.
 8. An apparatus for determininga depth of a ferromagnetic impurity in a workpiece of a nonmagneticmaterial comprising: means for magnetizing said ferromagnetic impurityuniformly by subjecting said workpiece at least in sections to amagnetic field; a magnetic field sensor scannable over said workpieceand movable relative to a surface of said workpiece for juxtapositionwith said workpiece at a first distance from said surface of saidworkpiece to obtain a first signal measuring magnetic field strengthderived from said ferromagnetic impurity after said ferromagneticimpurity has been magnetized in said magnetic field, and at a seconddistance from said surface different from said first distance to obtaina second signal measuring magnetic field strength derived from saidferromagnetic impurity; means for forming a signal ratio of said firstand second signals as a quotient of said magnetic field strength at saidfirst and second distances; and means for determining from a curve inwhich the dependency of the signal ratio is plotted against impuritydepth, based on said signal ratio of said first and second signals adepth of the impurity in the workpiece from which the magnetic fieldstrengths were measured.
 9. The apparatus defined in claim 8, furthercomprising a storage for a family of curves of different depths givingthe dependency between mass of the impurity and a measurement signal atdifferent depths, and means for calculating the mass of the impurityfrom the family of curves by selection of the curve corresponding to thedetermined depth and based upon the measurement signal.
 10. Theapparatus defined in claim 9, further comprising means for storing afamily of curves of different masses of impurity and giving a dependencybetween depth and measurement signal for different masses, and means forcalculating the mass of the impurity by selecting one of said curvesbased upon the determined depth and on the measured signal.